This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
Wireless communication systems typically deploy numerous base stations (or other types of wireless access points such as eNodeBs) for providing wireless connectivity to mobile units (or other types of user equipment). Each base station is responsible for providing wireless connectivity to the mobile units located in a particular cell or sector served by the base station. In some cases, the mobile units may initiate wireless communication with one or more base stations in the network, e.g., when the user of the mobile unit would like to initiate a voice or data call. Alternatively, the network may initiate the wireless communication link with the mobile unit. For example, in conventional hierarchical wireless communications, a server transmits voice and/or data destined for a target mobile unit to a central element such as such as a Radio Network Controller (RNC). The RNC may then transmit paging messages to the target mobile unit via one or more base stations. The target mobile unit may establish a wireless link to one or more of the base stations in response to receiving the page from the wireless communication system. A radio resource management function within the RNC receives the voice and/or data and coordinates the radio and time resources used by the set of base stations to transmit the information to the target mobile unit.
One alternative to the conventional hierarchical network architecture is a distributed architecture including a network of access points, such as base station routers, that implement distributed communication network functionality. For example, each base station router may combine RNC and/or PDSN functions in a single entity that manages radio links between one or more mobile units and an outside network, such as the Internet. Base station routers wholly encapsulate the cellular access technology and may proxy functionality that utilizes core network element support to equivalent IP functions. For example, IP anchoring in a UMTS base station router may be offered through a Mobile IP Home Agent (HA) and the GGSN anchoring functions that the base station router proxies by through equivalent Mobile IP signaling. Compared to hierarchical networks, distributed architectures have the potential to reduce the cost and/or complexity of deploying the network, as well as the cost and/or complexity of adding additional wireless access points, e.g. base station routers, to expand the coverage of an existing network. Distributed networks may also reduce (relative to hierarchical networks) the delays experienced by users because packet queuing delays at the RNC and PDSN of hierarchical networks may be reduced or removed.
At least in part because of the reduced cost and complexity of deploying a base station router, base station routers may be deployed in locations that are impractical for conventional base stations. For example, a base station router may be deployed in a residence or building to provide wireless connectivity to the occupants of the residents of the building. Base station routers deployed in a residence are typically referred to as home base station routers or femtocells because they are intended to provide wireless connectivity to a micro-cell (or femtocell) that encompasses a residence. However, the functionality in a home base station router is typically quite similar to the functionality implemented in a conventional base station router that is intended to provide wireless connectivity to a macro-cell that may cover an area of approximately a few square kilometers. One important difference between a home base station router and a conventional base station router is that home base station routers are designed to be plug-and-play devices that can be purchased off-the-shelf and easily installed by a lay person. Deployment of home base station routers may result in a very large number of femtocells, which may overlap with or be encompassed by one or more macro-cells.
Mobile units may be handed off from one base station to another as the mobile units roam throughout the wireless communication system. Mobile units may also be handed off from a macrocellular base station to a home base station router or femtocell, even when the coverage area of the base station completely encompasses the coverage area of the femtocell. For example, a user's mobile unit may hand off to a home base station router when the user returns home from work. From the point of view of the user, robust handover techniques are critical for supporting seamless service as the mobile unit moves around. Users quickly become frustrated by gaps or silences in voice communication that may be caused by latency in the handover process. Some users may even switch providers if calls are frequently dropped when the user roams from one cell to another.
The basic condition for initiating a handover is that the signal strength from the candidate target base station or cell is stronger/better than the signal strength from the current serving base station or cell. However, simply handing off a mobile unit as soon as the target base station appears to have a stronger signal than the serving base station can lead to a number of problems. For example, the signal strengths near the boundaries between a serving cell and its neighbor cells are (almost by definition) nearly equal. The signal strength received by each mobile unit near a boundary is therefore approximately equal and relatively small deviations can cause the relative signal strengths to flip-flop. The strength of the signals received by a particular mobile unit may also vary rapidly due to movement of the mobile unit and/or environmental changes. Consequently, the mobile unit may be rapidly handed back and forth (a phenomenon known as ping-ponging) if the hand off is performed based only on the relative signal strength. Ping-ponging consumes valuable overhead unnecessarily, degrades the perceived call quality, and can even lead to dropped calls.
Furthermore, the signal strength provided by a home base station router in its corresponding femtocell may not exceed the signal strength provided by a macrocellular base station at any point within the femtocell, particularly if the home base station router is deployed relatively close to the base station. In this situation, mobile units would never handoff to the femtocell if handoff was only triggered when signal strength provided to the mobile unit by the femtocell was larger than the signal strength provided by the macrocell. Consequently, femtocells would be nearly useless when deployed in areas that receive strong macrocellular signals. Handover to femtocells may therefore be triggered when the sum of a bias value and the signal strength received by the mobile unit from the femtocell is larger than the signal strength received from the macrocell. For example, the bias value may be set to 2 dB so that handover to the femtocell can be triggered when the femtocell signal strength is 2 dB less than the macrocell signal strength.